Device for distance measurement

ABSTRACT

The invention relates to a device with a detector element for receiving a light beam reflecting on an object, wherein means for detecting the light beam are provided in the detector element. The invention also relates to a method for distance measurement using said receiver device. A device for determining the distance between a vehicle and an obstacle or a vehicle driving ahead is already known as per DE 197 13 826 A1, wherein a pulsed laser beam serving as send signal scans line-by-line a two-dimensional area using a self-rotating polygonal mirror, wherein said polygonal mirror has a plurality of mirror surfaces that are bent at different angles. The pulse beam reflecting on the object is focused on a detector element by optical lenses. Said device known in prior art requires a polygonal mirror that is complicated to produce and a complicated evaluation electronics due to the fact that, inter alia, the scanning impulses for the control of the laser diode have to be synchronized with the angular position of the polygonal mirror. The invention provides for a self-rotating polygonal mirror with a plurality of mirror surfaces that are bent at a given angle against the rotational axis of said polygonal mirror as receiver device so that the reflecting light beam is deflected in said mirror surfaces by means of an optics system and focused on the detector element.

[0001] The invention relates to a device for measuring the distance toan object illuminated by a light beam in accordance with the preamble ofPatent claim 1.

[0002] Different methods are used in monitoring systems for motorvehicles for measuring the distances to obstacles or to vehiclestraveling ahead. These methods use for a measuring medium radarimpulses, microwaves, ultrasound waves, or infrared radiation. Asmeasuring methods for determining the distance, one can use either themeasuring of the transit time or one may use a triangulation method.

[0003] Thus, an apparatus is known, for example from German PatentPublication DE 197 04 220 A1 for determining the spacing of an obstaclefrom a vehicle in which a semiconductor laser, a radar transmitter, oran ultrasound generator is used for the emission of scanning impulses.The transmitted signal that has been reflected by an obstacle isdetected by a receiver apparatus. The distance to the obstacle isdetermined with a time measuring unit for acquiring the time between thesending of the transmitted signal and its detection point of time.

[0004] Furthermore, a radar apparatus for a vehicle safety spacingcontrol system is known from German Patent Publication DE 197 13 826 A1which performs a two dimensional scanning operation by using a rotatingpolygonal mirror in order to determine, for example relative data suchas a distance, a direction, or a speed of a vehicle traveling ahead. Inthis apparatus an infrared impulse beam is generated by a laser diode.This beam is reflected by a reflection mirror onto the rotatingpolygonal mirror. The polygonal mirror comprises a plurality of mirrorsurfaces that are inclined at different angles, so that thereby adetermined measuring range is scanned by the transmitter beam in aline-by-line fashion due to the differently inclined mirror surfaces ofthe polygonal mirror. In case an object is present in this measuringrange, the transmitter beam reflected there is detected by a detectorelement. The two-dimensional scanning operation takes place due to thefact that a pulse beam is directed onto each mirror surface of thepolygonal mirror. Thereby, the direction of each beam reflected by eachmirror surface is changed by the rotation of the polygonal mirror insuch a way that a horizontal scanning operation is accomplished in thehorizontal level. The scanning rate of a line can be increased byraising the impulse frequency. When the pulse beam is reflected by anext mirror surface, the horizontal scanning operation is repeated in adifferent elevational position which is proportional to the angle ofinclination of each mirror surface. This scanning operation is repeatedat different locations of the elevational direction, so that thereby atwo-dimensional scanning operation is realized. The number of differentelevational directions which thereby occur, that is the number of thelines, depends on the number of the mirror surfaces which are providedwith different inclinations relative to the rotation axis of thepolygonal mirror.

[0005] If such a measuring apparatus is used in a vehicle, it ispossible to adapt the scanning operation for the recognition of avehicle traveling ahead, to the incline characteristic of the road. Forexample, if the road rises ahead of the vehicle, the position of avehicle traveling ahead will also be higher than the position of thevehicle equipped with the measuring apparatus. In order to detect withcertainty the transmitter signal reflected by the vehicle travelingahead also in this situation the number of measurements made in therespective elevational direction is increased, i.e. the impulsefrequence of the transmitter signal is increased. In order to be able toperform such a control of the impulse frequency of the transmittersignal, an inclination angle sensor installed in the vehicle provides arespective measured value.

[0006] On the one hand the timed control which is expensive to realizeis a disadvantage in this known measuring apparatus. The timed controlrequires that the transmitter must produce an impulse beam with thecorrect impulse frequency in dependency on the rotation angular positionof the polygonal mirror. On the other hand a transmitter is requiredthat is capable of realizing the respective impulse frequencies.

[0007] Thus, it is the object of the present invention that the abovementioned apparatus for measuring the distance is improved with regardto a simpler evaluation of the detected light beams.

[0008] This object is realized according to the features of Patent claim1, wherein a rotating polygonal mirror is provided as a receiverapparatus of the light beam reflected by an object, said polygonalmirror comprising a plurality of mirror surfaces which are inclined at adetermined angle relative to the rotation axis of the rotating polygonalmirror, and wherein the reflected light beam is focused through such amirror surface onto the detector element by means of an optic allocatedto each mirror surface of the polygonal mirror and rotating insynchronism with the polygonal mirror.

[0009] With the apparatus according to the invention it is possible toderive, from the detected continuous light beam that has been reflectedby an object present within the measuring range and with the aid of thetransit time method, the distance as well as the associated position ofthe object on the basis of the angular position of the polygonal mirror.

[0010] The optic allocated to each mirror surface of the polygonalmirror comprises preferably a lens system particularly a single lens.Thus, a simple construction is achieved for the entire receiverapparatus. In a further preferred embodiment of the invention the opticsallocated to the mirror surfaces are positioned approximately along acircular line around the rotation axis of the polygonal mirror. Thereby,a screening is arranged between neighboring optics, that is, in the areaof the edges of the polygonal mirror. Thus, a defined opening angle isachieved so that the polygonal mirror which rotates together with theoptics and with the screens scans the measuring range with this openingangle, i.e. the range irradiated by the light source. Moreover, therebyit is assured that two mirror surfaces following each other sequentiallycannot be impinged upon simultaneously by reflected light beams.Depending on the desired resolution, the polygonal mirror according tothe invention can be equipped with three, four or five mirror surfaces.Thereby, it has been found to be advantageous to construct the screensrespectively in such a manner that an opening angle of 60° is produced.

[0011] A simple distance measuring method can be performed with theapparatus according to the invention in which a continuously emitted andmodulated light beam irradiates a spatial range that is to be monitoredas a measuring range, wherein the distance of the object can be derivedby a phase comparing with the detected light beam reflected by an objectand wherein the position of the object can be derived from the angularposition of the polygonal mirror.

[0012] A further improvement of the method is achieved in that the lightbeam is produced by modulating a pseudo noise signal. A goodinterference signal suppression can be achieved with a light beammodulated in this fashion in that the distance and the position of theobject which reflected the light beam is determined by correlationbetween the pseudo noise signal and the detected signal.

[0013] The invention will be explained in more detail in the followingwith reference to an example embodiment in connection with theaccompanying drawings.

[0014]FIG. 1 shows a schematic illustration of an apparatus according tothe invention for measuring the distance with a ray path of a light beamproduced by a transmitter apparatus;

[0015]FIG. 2 shows a top plan view of the apparatus according to FIG. 1;

[0016]FIG. 3 shows a side view of the apparatus according to FIG. 1; and

[0017]FIG. 4 is a schematic block circuit diagram for explaining thefunction of the arrangement with the apparatus according to theinvention as shown in FIG. 1.

[0018] The arrangement according to FIG. 1 shows an apparatus 1according to the invention for receiving a light beam 3 a produced by atransmitter 2. The light beam 3 a is directed onto a spatial range 4 andis reflected there by an object for determining the object's distance tothe receiver 1.

[0019] The transmitter 2 comprises a semiconductor laser functioning asa light source for producing an infrared light beam 3 a, which isdirected as a transmitter signal onto the spatial range 4 forming ameasuring range where the light beam is reflected by an object (notshown) of which the distance is to be determined, in order to then bedetected as a reflected light beam 3 b by the receiver 1. The receiver 1comprises as its central component a polygonal mirror 5 that rotatesabout a rotation axis 6 and has three mirror surfaces which, as shown inFIGS. 1 and 2, form an equilateral triangle. Furthermore, a lens system7 rotates with the polygonal mirror 5. The lens system 7 comprises threeoptical lenses 7 a, 7 b and 7 c which are respectively arranged oppositea mirror surface of the polygonal mirror 5. In the angular position ofthe polygonal mirror 5 shown in FIG. 1 the reflected light beam 3 b isfocused onto a detector element 9 by means of the lens 7 a and throughthe mirror surface of the polygonal mirror 5 positioned opposite thislens. The detector element 9 is arranged in the direction of therotation axis 6 of the polygonal mirror 5. For this purpose and as shownin FIG. 3 the mirror surface 5 a of the polygonal mirror 5 is inclinedin such a way that following the deflection of the reflected light beam3 b by this mirror surface 5 a this light beam 3 b impinges on thedetector element 9. The other two mirror surfaces 5 b and 5 c of thispolygonal mirror 5 have the same inclination so that upon rotation ofthis receiver apparatus the reflected light beam 3 b will also bedeflected by these other mirror surfaces onto the detector element 9.

[0020] As shown in FIG. 2, respective screens 8 are arranged along acircular line around the polygonal mirror 5 between the optical lenses 7a, 7 b and 7 c in order to thereby scan the measuring range 4 with adefined opening angle α. Without these screens 8 there would be anopening angle of 120° due to the cross-section of the polygonal mirror 5forming a unilateral triangle. Thereby, it would then be a disadvantagethat for a certain angular position of the polygonal mirrorsimultaneously two neighboring mirror surfaces could receive a lightbeam. Thus, it is advantageous to construct these screens 8 in such away that the opening angle α is, for example 60° . Due to the rotationof the receiver 1 the space range 4 is scanned with an opening angle αwhereby the reflected light beam is focused onto the detector element 9.The distance of an object is determined from the received signal of thedetector element 9, for example by means of a phase transit time method.The associated position of the object is determined from the angularposition of the polygonal mirror. The resolution capability of thisarrangement can be increased if, instead of the three sided polygonalmirror 5 a polygonal mirror is used with four or five mirror surfaces,whereby simultaneously an opening angle of also 60° can be realized withthe described screens 8.

[0021] If the arrangement according to FIG. 1 is arranged in the area ofthe radiator cover of a motor vehicle, it is of advantage that thetransmitter 2 is oriented in such a way that the light beam 3 a isdirected onto the measuring range 4 with an opening angle of about 45’0and an angle of about 8° in the vertical direction.

[0022] A method for distance measuring by using the arrangement of FIG.1 shall be described with the block circuit diagram according to FIG. 4.As has been mentioned already above, the transmitter 2 comprises asemiconductor laser diode or an infrared transmitter diode with acontrol circuit. For controlling such a diode 12 of the transmitter 2for the production of a continuous and modulated transmitter signal, acarrier signal with a frequency f₀ is produced by an oscillator (L.O.)10 as a signal source. The carrier signal is modulated by a modulator(M1) 11 corresponding to a pseudo-noise sequence generated by apseudo-noise signal generator (PRS-G) 13. The transmitter diode 12 ofthe transmitter 2 is triggered with this carrier signal thus modulated,whereby the transmitter diode 12 produces a pseudo-noise signal as atransmitter signal which is directed as the light beam 3 a onto themeasuring range 4.

[0023] The signal reflected as light beam 3 b is focused onto thedetector element 9 by means of the optical lens 7 and through thepolygonal mirror 5. The detector element 9 produces a correspondingreceived signal.

[0024] This received signal is amplified, if necessary, and supplied toa mixer (M2) 16 for mixing with the pseudo-noise signal of thepseudo-noise signal generator 13. The pseudo-noise signal has beendelayed by means of a delay circuit 14. The thus produced signal is fedto a correlator 17 for determining the correlation between thetransmitted signal and the received signal. The result of thecorrelation from the correlator is evaluated by an evaluating unit 18for determining the distance. The evaluating unit 18 also determines thedirection of an object detected in the measuring range 4, together withthe angle of the angular position of the receiver 1 acquired by adetector 19 according to FIG. 1.

[0025] The pseudo-noise signal train generated by the pseudo-signalgenerator 13 serves, in addition to producing the modulated transmittersignal, simultaneously also as a reference signal which is compared withthe received measured signal. The result of this comparing provides aquantitative information regarding the distance to the detected object.The use of the auto-correlation for the measuring technical evaluationof the received light signal leads, due to the high filtering effectregarding noise signals, particularly in connection with the use in thefield of motor vehicles, to a high non-susceptibility to interferencebecause interference signals which are produced by devices of similarconstruction in the motor vehicle field typically have little similaritywith the used measuring signal.

[0026] Thus, the arrangement according to FIG. 1 in combination with thedescribed measuring principle of the auto-correlation can be used withadvantage not only for measuring the distance to a vehicle moving ahead,but also for the closed loop distance control or in precrash systems.

1. An apparatus (1) with a detector element (9) for receiving a lightbeam (3 a, 3 b) reflected by an object, whereby means (7, 5) areprovided for deflecting the reflected light beam (3 b) onto saiddetector element (9), characterized in that a rotating polygonal mirror(5) is provided as a means for deflecting the reflected light beam (3b), said polygonal mirror having a plurality of mirror surfaces (5 a, 5b, 5 c) inclined at an angle (β) to the rotation axis (6) of therotating polygonal mirror (5), whereby the detector element (9) isarranged in the direction of the rotation axis (6) of the polygonalmirror (5), and that an optic (7) is allocated to each mirror surface (5a, 5 b, 5 c) of the polygonal mirror (5) for focusing the reflectedlight beam (3 b) onto the detector element (9), said optic rotating insynchronism with the polygonal mirror (5).
 2. The apparatus of claim 1,characterized in that the optic (7) is constructed as a lens system. 3.claims 3 to 7 (canceled).
 8. The apparatus of claim 1, characterized inthat the optics (7 a, 7 b, 7 c) are arranged approximately along acircular line around the rotation axis of the polygonal mirror (5) andthat screens (8) are provided between neighboring optics (7 a, 7 b, 7 c)on the circular line.
 9. The apparatus of claim 1, characterized in thatthe polygonal mirror (5) comprises at least three mirror surfaces (5 a,5 b, 5 c).
 10. The apparatus of claim 8, characterized in that for apolygonal mirror (5) with three, four or five mirror surfaces (5 a, 5 b,5 c), the screen (8) produces for the mirror surfaces (5 a) a respectiveopening angle (α) of 60°.
 11. A method for distance measuring by meansof the apparatus according to claim 1, characterized in that acontinuously emitted and modulated light beam (3 a) irradiates a spacerange (4) to be monitored, and that by phase comparing of this modulatedlight beam (3 a) with the detected light beam (3 b) the distance to anobject present in the space range (4) to be monitored is determined andwith the angular attitude of the polygonal mirror (5) the position ofthe object is determined.
 12. The method of claim 11, characterized inthat a pseudo-noise signal train is used for modulating the light beam(3 a).